Interesting thread, been following it from over here in Glastonbury. Not Phoenix obviously but I had something similar a few months back - triangular, slow moving, no sound at all which is what got my attention.
Quick question for those who actually saw it - did it have lights at each corner or more like a strip of light along the edges? Mine had three distinct points of light and I couldnt work out if it was one craft or three separate objects flying in formation. I've got photos but they're not great, older camera and I wasn't expecting anything so the settings were all wrong.
Also curious whether anyone tried contacting MUFON about this one yet. I always wonder if its worth the bother but a multiple witness sighting over a city that size seems like something they'd actually look at properly.